1. Field of the Invention
The present invention relates to a method and a circuit of erasing a flash memory cell which can minimize the occurrence of an instantaneous peak current occurring at the beginning of erasing operation of flash memory cell.
2. Description of the Prior Art
FIG. 1 shows a split gate type flash memory cell to illustrate a conventional method of erasing a flash memory cell.
To erase the flash memory cell, 0V is applied to a select gate electrode 4 of the flash memory cell, -12V is applied to a program gate electrode 5, a source electrode 1 is floated, 0V is applied to a P-type substrate 3 and a source voltage Vcc is applied to a drain electrode 2. At this time, electric charges accumulated in a floating gate electrode 6 are discharged to the drain electrode 2 by F-N tunneling so that the flash memory cell performs the erase operation.
FIGS. 2A and 2B are waveform diagrams showing variances of voltages supplied to respective electrodes and of corresponding initial currents at the time of conventional erasing operation. Here, reference numeral E represents an erasing operation voltage, Q1 represents a program bias voltage supplied to the program gate electrode, D represents a drain bias voltage supplied to the drain electrode and Q2 represents a bias voltage for erasing. In FIGS. 2A and 2B, t1 represents an entire erasing operation time, t2 represents an initial erasing operation time, t3 represents an erasing operation time, t4 represents a read operation time, and t5 represents an erasing confirmation operation time, and it can be noted that the instant peak current steeply rises at the initial erasing operation time t2.
In such a conventional erasing circuit, if a plurality of flash memory cells are simultaneously erased, a large current initially flows due to a tunneling phenomenon between the program gate electrode 5 and the drain electrode 2 so that a power of a device, that is, the instant peak current at the source voltage Vcc increased large. And there is a disadvantage that the power of a system using the flash memory device is accordingly influenced, which in turn influences other devices.